Slate.com has a good overview of the recent outbreak of mumps in the midwest and discusses one potential cause of the outbreak -

No vaccine is perfect, however. The worst mumps epidemic in 17 years
is currently sweeping through a heavily vaccinated population in the
Midwest. Nearly 1,000 cases have been reported in Iowa since March, and
hundreds more in seven other states. A single dose of mumps vaccine—the
recommended dosage in Iowa before 1991—protects about 80 percent of
those who get it; two doses, the norm today, confers 90 percent
immunity. The Iowa outbreak is concentrated in college students, about
one-third of whom had not gotten two doses of vaccine as children.
Mumps is more serious in older kids and adults—about a fifth of
infected men get swollen testicles, as well as the disease's trademark
symptom of a swollen neck; women get sore breasts. Outbreaks in
vaccinated populations tend to be less serious, though, and only about
20 people have so far been hospitalized, according to Julie Gerberding,
director of the Centers for Disease Control.

The CDC is
investigating the origins of the Midwest outbreak. The agency has no
clear answers yet, but it may be that these mumps cases are linked to
an earlier vaccine scare in Britain. In 1998, gastroenterologist Andrew
Wakefield published a report in the medical journal Lancet in
which he suggested that the MMR vaccine (called that because mumps
protection is nearly always packaged together with vaccines for measles
and rubella) caused autism. The article has since been withdrawn and
the link between the vaccine and autism disproved. But vaccination
rates plunged in the United Kingdom for a few years, resulting in a
mumps epidemic of 100,000 cases since 2004. The mumps virus circulating
in the Midwest is the same type as the British epidemic strain, though
it is too common a variant for scientists to be certain, at this point,
of a definitive link.